Controlling wastewater treatment by monitoring oxygen utilization rates

ABSTRACT

A method and apparatus for treating waste material to remove selected components form the waste is described using a reactor or a series of reactors in fluid communication with each other for receiving the waste to be treated as influent. The influent forms a biomass including the waste and microorganisms and is treated by controlling the metabolic activity of the microorganisms by monitoring the oxygen utilisation rate or the potential oxygen utilisation rate of the biomass so as to determine the required amount of oxygen to be supplied to the biomass and to determine the period of aeration of the biomass in order to maintain a predetermined oxygen utilisation rate or value so as to remove the selected components of the waste. The preferred selected components to be removed are nitrogenous, carbonaceous and/or biological phosphorus containing materials or derivatives.

This invention relates to improvements in wastewater treatment generallyand in particular to wastewater treatment methodology usingmicro-organisms and means of controlling the metabolic activity of thosemicro-organisms in a variable volume activated sludge reactor which isintermittently, aerated and decanted. More particularly, the presentinvention relates to methods and apparatus for controlling the metabolicactivity of dispersed growth micro-organisms through the regulatedsupply of oxygen relative to in basin biomass oxygen uptake ratemeasurements, to achieve the beneficial result of carbon or carbonaceousmaterial removal, as measured by COD, BOD, TOC; nitrogen removal asmeasured by TKN, NH₃ --N, NO₂ --N, NO₃ --N; and phosphorous removal, asmeasured by PO₄ from a wastewater. The present invention findsparticular application in treating domestic wastewater, industrialwastewater or a mixture thereof. The invention particularly relates tomaximising the rate of removal of biologically degradable materials in awastewater by micro-organisms by optimising the metabolic activity ofthe micro-organisms that are used in a single sludge reaction procedure.In so doing it in recognised that there are at least four major speciesor families of micro-organisms in the overall biological consortia thatneed to be maintained. Those micro-organisms that are generallyresponsible for the net removal of carbohydrate type compounds, thosemicro-organisms that generally oxidise nitrogen compounds to nitratenitrogen, those micro-organisms that generally denitrify nitrate tonitrogen gas and those micro-organisms that generally participate inenhanced biological phosphorus and in the overall hydrolysis ofdegradable volatile solids to soluble degradable substrate. UP to 20000individual species of micro-organisms can be contained in the overallconsortia constituting the biomass.

Although the present invention will be described with reference to thetreatment of industrial wastewater and domestic wastewater and to themethodology of such treatments, it will be appreciated by one skilled inthe art that the invention is not limited to such applications and maybe used to treat any type of biologically degradable wastewater orotherwise, and any type of waste including water or waste having thespecific impurities or contaminants as discussed herein.

Conventional activated sludge processing requires detailed monitoringinformation on which to base process control decisions to meet treatmentobjectives. These analyses, which are well known to practitioners of theart, typically include BOD (total), COD (total), BOD (soluble), COD(soluble), TKN, ORG--N, NO₃ --N, ortho Phosphate, total Phosphate, pH,Alkalinity for both influent and effluent streams. In basin measurementsinclude, Dissolved Oxygen Concentration, Mixed Liquor Suspended SolidsConcentration, Mixed Liquor Volatile Suspended Solids Concentration,Sludge Settled Volume, Biomass Degradable Fraction (through aerobicdigestion of the biomass for 28 days). Simple parameters, incorporatingthe Potential Oxygen Utilisation Rate (POUR) and its actual utilisationrate are used for the automatic control and operation of a singleactivated sludge variable volume reactor in order to achieve a very highdegree of carbon, nitrogen, phosphorus removal without sludge bulking.

The present invention relates to activated sludge wastewater treatment,the principal reactor of which is configured for complete-mix operation.While variable volume intermittently aerated and decanted fed-batchoperation can be used as the preferred embodiment, the technique alsoapplies to constant volume continuously aerated complete-mix operation.The keywords are fed-batch, intermittently aerated, complete-mix,reactor basin. In this invention there maybe a series of activatedsludge reactors all connected by conduit pipe or other means, with orwithout means for flow interruption between the said reactors. The lastreactor in each series of reactors is termed the principal reactor fromwhich the biologically treated effluent is directed. It will be apparentto those skilled in the art, that the reactor may be formed as a slopewalled lagoon structure, with earthen, concrete stabilised, membranelined or concrete retaining walls, or as a conventional reinforcedconcrete walled vessel or as a structural steel vessel. While someshapes, and dimensioning ratios of the basins may be preferred it isimportant to state that any geometrically shaped vessel (square,rectangular, circular) can be operated in accordance with this inventionspecification.

It is well known by those experienced in the art that a number ofreaction conditions need to be satisfied in order to achieve biologicalnitrification--denitrification and enhanced biological phosphorusremoval. In particular the nitrification reaction requires an adequatesupply of inorganic carbon. The removal of phosphorus by biologicalmeans requires selectivity reaction circumstances to cause the necessarymicro-organisms to proliferate. Among chose requirements is a substratepreferably containing volatile fatty acids and more commonly referred toas readily degradable soluble substrate. Additionally required arereaction conditions that cycle between the so called description oftoxic and anaerobic. It is necessary to be more definitive when usingthese terms as there are degrees of anaerobicity which trigger certainbiological reactions. An absence of oxygen and nitrite--nitrate is incurrent terminology not sufficient to describe "anaerobic" to the extentthat biological phosphorus removal will take place. Anaerobic reactionconditions require a more exacting definition when applied to phasedactivated sludge processing whereby oxic, anoxic and anaerobic reactionconditions can be induced on a single sludge culture by relativelysimple manipulation of fill and aeration sequencing. Selectivitypressures are dominated by exposure of the culture to highacetate--substrate loading pressures under sequenced anaerobic, anoxicand oxic reaction conditions. An absence of nitrate and dissolved oxygenconcentration is not sufficient to define anaerobic conditions whichwill cause the relevant micro-organisms species to release its contentof Poly P. According to conventional knowledge it is usual to describeappropriate reaction conditions in terms of a bulk liquid oxidationreduction potential (a value of EMF referenced to a standard electrodemeasurement of hydrogen or silver chloride). Hence this value needs tobe sufficiently negative (-150 mV, Hydrogen electrode reference) toensure a degree of definable anaerobicity to ensure the phosphaterelease mechanism. It has been found that the ORP depletion rate frompositive (oxidizing) conditions to negative (reducing) conditions isfunctional on the metabolic activity of the biomass at the switchingoxidation reduction potential. The same metabolic activity is a functionof the amount of residual intracellular storage compounds maintained inthe culture. Using this description, a biomass having a high value ofoxygen uptake rate in an oxidation environment will rapidly approachmore negative ORP values when the oxidizing reactant (oxygen) isremoved. A biomass having a lower value of oxygen uptake rate willconsequently deplete its ORP at a slower rate. Biological phosphorusrelease occurs at a value some 250 mV more positive than the values thatequate to the reduction of sulfate to sulfide. In the practice of theart, using other conventional constant volume processing, it has beennecessary to define hydraulic retention time criteria as a means ofensuring appropriate reaction conditions. Through research and trial anderror, a range of parameters has been found, relating to process andsimply described in terms of the actual oxygen uptake rate of the singlesludge biomass that can be used to specify the reaction conditions thatensure a reliable and continued desirable process result. Theapplication of these control parameters to the operation of thepreferred embodiment provides an overall process that is less expensivethan the generally accepted conventional methodology and one which ismuch less complex to operate. The principal parameter relates to anoverall activity level of the biomass as measured by its oxygenutilisation rate (OUR) and its potential oxygen utilisation rate (POUR).Process control using these parameters enables the use of set pointvalues which obtain the reliable removal of pollutants and nutrients andat the same time produce a biomass which has excellent solids--liquidseparation properties.

Therefore, it is an aim of the present invention to provide a method andapparatus for treatment of wastes which at least alleviates one or moreof the problems of existing methods and apparatus by more closelymonitoring process conditions and parameters relating to the activity ofthe biomass, such as for example oxygen utilisation rates includingpotential oxygen utilisation rates.

According to the present invention there is provided a method oftreating waste by controlling metabolic activity of micro-organisms of abiomass containing the waste so as to remove selected components of thewaste prior to disposal of treated waste, characterized in that themethod comprises monitoring at least one oxygen utilisation rate of thebiomass in order to determine a requisite amount of oxygen to besupplied to the biomass and monitoring a period of aeration of thebiomass by the oxygen so as to maintain a predetermined oxygenutilisation rate or value to achieve removal of the components.

One aspect of the present invention relates to the sizing of theactivated sludge reactor(s), their mode of operation and the automaticoptimisation of the amount of oxygen supplied to the reactor(s) in termsof rate and time of application by sensing the metabolic activity of thebiomass in the principal reactor. This metabolic activity is sensed asthe actual oxygen utilisation rate of the biomass in the principalreactor near to the and or at the end of an air-on sequence. Uponinterruption of the supply of air to the principal reactor, the contentstherein remain in motion for up to ten minutes, the natural mixingmotion increasingly decreases with time. Values of dissolved oxygenconcentration are sensed and monitored at intervals of ten or twentyseconds. A minimum of ten points are taken and mathematically treated toprovide a slope of best fit which best describes the initial dissolvedoxygen depletion rate and hence a nominal actual oxygen utilizationrate. These data are trend plotted with cycle volume, the volumetricload, pertaining to the activity measurement plus the maximum dissolvedoxygen concentration sensed during the cycle. The sensed dissolvedoxygen concentration and blower speed profile is also recorded. Theinvention relates to the maintenance of a biomass (mixed culture ofmicro-organisms), through optimal oxygen input, having a selectableoptimal biological activity as measured by its oxygen utilisation rate,volatile suspended solids fraction and degradable volatile suspendedsolids fraction as later defined. The dissolved oxygen sensor measuresthe in-situ biomass oxygen utilisation rate for use in controlling andregulating the input of oxygen from the air input device pump orcompressor. As described with reference to the preferred embodiments,reaction conditions in this principal reactor variously sequence fromair-on to air-off. An air-on sequence will typically be continuous andoccurs while influent wastewater is introduced into the basin(s) thenstops during which time the biomass in the principal treatment reactorsettles after which supernatant clear liquid is removed from theprincipal process reactor. The invention operates similarly withnon-continuous air on sequencing. When the effluent removal sequence iscompleted air and untreated wastewater is again introduced into theprincipal process reactor until the air sequence is again stopped. Atotal cycle of operation can typically be four hours, an aerationsequence will typically be two hours; other time combinations can beused. To those skilled in the art it can be easily seen that other timeincrements may be used. Two measurements are made. The rate of depletionof dissolved oxygen during the initial minutes after the cessation ofaeration. Other intermediate rates associated with multiple aerationsequencing can also be read. A second rate is measured when the air isagain turned on during which time a maxaimum flow rate of air isintroduced into the reactor or a section of the reactor for a set time(this in a variable which needs to be set for each plant and be subjectof relatively infrequent adjustment through a check calibrationprocedure. The rate of change of dissolved oxygen (dO₂)/dt increase anddepletion, and the manner in which the biomass settles d(MLSS)/dt arerelated where O₂ refers to concentration of dissolved oxygen and (MLSS)refers to simple concentration of activated sludge. Both vary with timewhen the introduction of air to the basin is stopped. Similarly there isa time variation of both parameters during the initial period of anaeration sequence. In the preferred embodiment the principle reactor ofthe system is configured with diffuser grids and feed lines to providemore than one effective mixed reaction zone upon introduction of air. Aminimum of one section of the principal reactor will typically beaerated at the start of an aeration sequence. Biomass from this initialaerated mixed zone is used to determine the rate of change of oxygenincrease at the start of an aeration sequence. In the preferredembodiment it is possible to time select the various grid zones foraeration. In those embodiments that have a single grid assembly, thesame results will be obtained through aeration of the total principalreactor volume.

A part of the invention lies in the in-basin measurement of oxygenutilisation rate in order to provide the requisite oxygen in terms ofrate of supply and period of aeration, to maintain a set point oxygenutilisation rate. This in turn sets the reaction conditions for theprocessing of wastewater using fed-batch single sludge single reactortechnology. Measurement and control is but one part of the invention.The reactor basin processing, as described by the preferred embodiment,is closely associated with the measurement aspect. Both are cognated inthe present invention. It will be understood by those experienced in theart that aeration of the principal reactor for too long, in successivesequences, will quickly lead to a loss of metabolic activity of thebiomass therein and a subsequent inability of that biomass to properlydenitrify and to take part in the removal of phosphorus by biologicalmass. Over aeration of the biomass will also lead to a reduced flocaggregation and hence an undesirable increase in effluent suspendedsolids concentration. Continued operation outside of the desired sludgeage envelope will lead to a similar consequence. Biomass oxygenutilization rate measurement is used to fix the envelope of operatingsludge age.

The present invention will now be described by way of example withreference to the accompanying drawings in which:

FIG. 1 is a schematic view of one form of the reactor of the presentinvent-ion having a single reactor divided into two compartments;

FIG. 2 is a schematic view of another form of the reactor of the presentinvention being a single basin configuration having a main reactor andseparate auxiliary reactors;

FIG. 3 is a schematic view of one form of an intra flocanoxic-denitrification model used in the present invention;

FIG. 4 is a plot of biorate feed-starve set point operation;

FIG. 5 is a schematic diagram showing definitive conditions of oxic,anoxic and anaerobic reaction conditions expressed in terms of bulkphase measured oxidation reduction potential;

FIGS. 6(a) to 6(g) are schematic views of alternative forms of thereactor showing different configurations for feed inputs and effluentoutlets, including multi-split inputs and outlets.

While it will be realised by those experienced in the art, that thereaction embodiment can take a number of forms, a simple embodiment forthe purposes of instruction will now be described.

In FIG. 1 is shown schematically one form of a single basin reactor ofthe present invention. The boundary of the reactor basin shown in FIG. 1is shown in elevation and is depicted as (1) being of solid constructionand designed to contain water. A minimum of two reactor zones, shown as(3) and (4) caused by a sub compartment, partial wall, baffle wall orthe like (shown as (2)), is depicted. The reactor zones are in fluidcommunication by pipe or other conduit or by a section of open areaformed by the partial baffle wall. Means for diffusing air for thereactive oxygen component, preferably by a grid of membrane diffusers,is shown as (5) receiving a flow of compressed air from a mechanicalengine shown as (6). A means for transferring the contents of (4), theprincipal reactor, using a regulating transfer pump to come in contactwith the influent flow (designated (11)) and for its admixture andreaction in (3) is shown. Two important levels are shown in the reactorbasin, that of (8), the designated bottom water level and that of (9),the designated top water level. In this embodiment a sequence ofaeration takes place while flows designated (10) and (11) take place,i.e. filling from bottom water levels (8) to top water level (9). Whenthis sequence is complete, the means of aeration are interrupted to stopthe mixing and oxygen transfer procedure thus allowing the mixed solidsto settle and separate to form an overlaying supernatant clear layer ofliquid on top of a layer of settled solids. At an appropriate tine thedecanter (9) is caused to function and to remove the volumetric depthbetween (8) and (9), after which its functionality ceases until the endof the next cycle. In this embodiment in flow (11) may be continuous orintermittent; outflow through the operation of the decanter (9) isnecessarily discontinuous relative to the total time span of the cyclethat permits the operation of inflow and aeration, settle and decant.The placement of a dissolved oxygen sensor (12) either within theprincipal reactor (4) or within the pumped line feeding biomass from theprincipal reactor for admixture of influent (11) within the initialreaction zone (3), is marked 14. An instrument that can be used formonitoring the in-basin concentration of the biomass (mixed liquorsuspended solids) shown as (13) may be used in the preferred embodiment.A sludge blanket interface detector (15) is also useful for automaticsludge wasting operation from the preferred embodiment. Two floormounted diffuser grid assemblies are shown; (16) and (17) schematicallyshow means for selectively using a grid assembly which is constituted bymore than two downcomer--valve attachments. It will be seen by thosethat are experienced in the art that a principal reactor basin may havemany more than two downcomer valve attachments, depending upon the totalarea of the reactor basin and the effective area of influence of themeans for diffusion mixing and oxygen transfer. Reactor embodimentsprovide for selective and sequenced area aeration or for total areaaeration.

The embodiment of the reactor(s) of the present invention shown in FIG.2 has similar components to the reactor of FIG. 1 and accordingly thesame reference numerals are used to identify similar features of thereactor(s).

The present invention relates to wastewater treatment methodology andmeans of controlling the overall metabolic activity of dispersed growthmicro-organisms within a single sludge mass to achieve the beneficialresult of reliable simultaneous carbonaceous removal, as measured byCOD, BOD, TOC, nitrogen removal, as measured by TKN, NH₃ --N, NO₂ --N,NO₃ --N, and phosphorus removal, as measured by PO₄, from a wastewaterand within the time frame of a repetitive cycling of process events. Theinvention relates to means of measuring in-basin oxygen utilisation rateand manipulating aeration input to maintain a set point regime ofreaction conditions that will permit single sludge single basintreatment for carbon removal and/or nitrogen removal and/or-enhancedbiological phosphorus removal. These reaction conditions are dependentupon a set point oxygen utilisation rate as it deternines the viabilityof the microbial population at the set operating sludge age and isdeterministic on the not settling properties of the single sludge.Wastewater may be essentially domestic or industrial or a mixture ofboth types.

industrial wastewater is described as a discrete classification todifferentiate from total household wastewaters which essentiallycomprise human wastes (faeces, urine), body washing wastewater, clotheswashing wastewater and food preparation wastewater. Industrialwastewaters are essentially those wastewaters that are generated in themanufacture of products and in particular are wastewaters that arebiodegradable. State-of-the-art technologies using dispersed growthmicro-biological reactions have been well described in the literature,for example:

Quirk T., Eckenfelder W. W., and Coronszy M. C., "Activated Sludge;State-of-the-Art". Critical Reviews in Environmental Control, CRC PressVol. 15, Issue 2, 1985.

Eckenfolder W. Wesley, Jr. "Industrial Wastewater Treatment" McGrawHill, 1991.

Eckenfelder W. Wesley, Jr. "Principles of Water Quality Management"C.B.I. Publishing Company, Inc., 1980.

Without limiting the coverage of the invention, reference is made tofractional components of a wastewater; the relative factions may bedifferent in domestic and industrial wastewaters. It is important torecognize that those fractions exist and their relative magnitude canimpact upon the methodology of using the invention and the processconfiguration in which that invention is embodied.

It is important to recognise that wastewaters typically comprise solubleand insoluble components which include readily degradable solubleorganics, degradable soluble organics that are not as rapidlydegradable, non degradable soluble organics, readily hydrolysable anddegradable particulate substrates, slowly degradable particulate and nondegradable particulate substrates. These substrates, their relativeconcentrations and their relative concentrations to other componentssuch as TKN, NH₃ --N, NO₃ --N, total P and ortho P may have a largeinfluence on the rate and generation of certain dispersed growthmicro-organism species.

Goronazy M. C. and Eckenfelder, W. W., "The rate of the degradation ofprimary solids in activated sludge plants" Proceedings Water PollutionControl Federation Conference, Toronto, Canada. October 1991.

Dispersed growth wastewater treatment methodology typically involvesoxic, anoxic and anaerobic reaction environments and mechanisms throughwhich energy transformations take place involving electron acceptors togenerate a net reduction in concentration of organic compounds asmeasured by BOD, COD, TOC and nitrogen and phosphorus (FIG. 5).

These regimes of processing can be generally described in part throughthe concentration of dissolved oxygen, nitrite and nitrate nitrogen,sulphate, phosphate and in part through the scale of oxidation reductionpotential (ORP) relative to the standard hydrogen electrode. Positivevalues of ORP typically relate to oxidative conditions while negativevalues of ORP typically relate to reducing conditions. There is nodefined relationship between ORP and dissolved oxygen concentration onthe positive scale, although the input of oxygen as a chemical source ofoxygen will cause a response in ORP to be less negative or morepositive. Temperature can influence the relative value of ORP as can thepresence and relative density of micro-organism species. Essentially theremoval of carbon compounds and TKN compounds requires aerobicconditions, the removal of NO3-N, and N2-N requires anoxic to anaerobicconditions and the removal of P requires oxic-anoxic and anaerobicconditions with cyclic exposure of the biomass, or specified fractionsof the biomass in the aeration basin, to achieve ORP reactionenvironments that vary between circa 50 mV to -150 mV (hydrogenelectrode reference) to enable all of the processing reactions to takeplace. The understanding of the actual discrete mechanisms, while beingimportant to treatment results is not important to the description ofthe preferred embodiment of the invention herein.

Suffice to say there are reaction regimes herein which provide anenvelope of performance which is necessarily required to permit thesingle sludge removal of the herein beforementioned parameters. Typicaldomestic wastewaters are described by 24 hour flow weighted compositesamples in which the measured parameters of total COD, TKN, Phosphorusare up to 1000 mgL⁻¹, 85 mgL⁻¹ and 15 mgL⁻¹ etc.

                  TABLE 1                                                         ______________________________________                                        Concentrations of Selected Constituents in                                    Municipal Wastewaters                                                                     Concentration (mg/L) related to                                               wastewater strength                                               Constituent Strong       Medium  Weak                                         ______________________________________                                        (a) BOD     400          220     110                                          (b) COD     1000         500     250                                          (c) SS      350          220     100                                          (d) Nitrogen                                                                  Total       85           40      20                                           Organic     35           15      8                                            Ammonia     50           25      12                                           Nitrite     0            0       0                                            Nitrate     0            0       0                                            (e) Phosphorus                                                                Total       15           8       4                                            Organic     5            3       1                                            Inorganic   10           5       3                                            (f) Alkalinity                                                                            150          100     50                                           (as CACO.sub.3)                                                               ______________________________________                                    

The relative amounts of carbon, nitrogen and phosphorus indicated byliterature values in Table 1 differ considerably from those required fornormal biological growth as reflected in the proportion of carbon andnitrogen given by the empirical analysis for cell material --C₅ H₇ NO₂-- together with the fact that cells contain around 1 to 2% ofphosphorus by mass. That is, carbon is present in short supply relativeto nitrogen and phosphorus in raw sewage as illustrated by Table 2. Thisshortage is worse for settled sewage and is further compounded by thefact that about 50% of the organic carbon is oxidised to CO₂ inbiological treatment.

The nitrogen and phosphorus in excess of biological requirementsnormally remain in the biological treatment plant effluent. The form inwhich these nutrients are present in the effluent may differ markedlyfrom that in the influent.

Nitrogen is present in raw sewage mainly as organic nitrogen andammonia, much of which results from hydrolysis of urea, a majorconstituent of urine. in biological treatment some of this nitrogen isincorporated into new cell growth and is removed as biological sludgewhile most of the remaining nitrogen may be either in the form ofammonia or, depending upon conditions in the plant, as the oxidisedform, nitrate, and to a lesser extent nitrite. Some organic nitrogenalso remains in the effluent, mainly in association with the effluentsuspended solids.

                  TABLE 2                                                         ______________________________________                                        Nutrient Imbalance in Municipal Wastewaters for                               Medium Strength Wastewater                                                               Relative Nutrient Proportions                                                 Carbon     Nitrogen    Phosphorus                                  Constituent                                                                              (mg/L)     (mg/L)      (mg/L)                                      ______________________________________                                        Typical    60         14          2.8                                         Biomass                                                                       (C.sub.5 H.sub.7 NO.sub.2,                                                    & p = .sup.N /.sub.5)                                                         Wastewater BODS =     NH.sub.4 - N =                                                                            10                                                     220        25                                                                 BOD.sub.ult =                                                                            Org. - N = 15                                                      323        Total N =                                                          c =        40                                                                 120                                                                Uptake in  60         14          2.8                                         CELL GROWTH                                                                   (Net Yield =                                                                  0.5)                                                                          gcellC /                                                                      GwasteC                                                                       Residual   --         26          7.2                                         Effluent                                                                      Concentration                                                                 (mg/L)                                                                        Overall                                                                       Removal (96)                                                                             100%       35%         28%                                         ______________________________________                                    

Phosphorus is present in raw sewage in two major forms--organic andinorganic. There are in fact many forms of phosphorus compounds in rawwastewaters, either in solution or in suspension. Inorganic dissolvedforms consist mainly of orthophosphates and condensed phosphates whilethe dissolved organic forms are organic orthophosphates.

One of the specific mechanisms concerns reaction conditions whichmaximise the initial rate of removal and storage of the readilydegradable soluble fraction of the influent wastewater flow entering thetreatment plant. The treatment plant is herein described as means toreceive said wastewater, means of contacting the influent flow ofwastewater with the manufactured active micro-organisms, means tocontain said wastewater in contact with the degrading micro-organisms toeffect the envelope of performance and means for separating the saidtreated wastewater from the degrading and remaining micro-organisms. Theenvelope of performance concerns the manufacture or presence of asufficient concentration of active micro-organisms (Xo) such thatintimate contact of these micro-organisms with the influent wastewaterreadily degradable soluble substrate (So) causes a rapid enzymaticreaction whereby the So is transferred into the bacterial culture withthe subsequent generation of PHB, glycogen and/or other intermediate`storage` compounds within the cell structure of the reactivemicro-organisms with a subsequent generation of glycocalyx (acoagulating polysaccharide compound). The transfer of substrate from theliquid phase to the solid phase is energy demanding. Under measurableoxic reaction conditions there is a rapid increase in the rate of demandfor using dissolved oxygen (its oxygen utilisation rate). The energyoxygen equivalence can easily be measured by introducing a mass ofdissolved oxygen to the biomass, the rate of utilization is measuredthrough simple dissolved oxygen versus time measurements. As therelative magnitude of the ratio So to Xo increases, the peak oxygenutilisation rate increases until a maximum or plateau value is reached.This is the first reaction envelope which also specifies a mass and rateof removal of readily degradable soluble substrates. The rate ofutilization of oxygen also parallels the rate of removal of liquid phasesoluble substrate and this allows the energy inter-relationship to beformulated (FIG. 3).

The measurement of degradation of a wastewater using an oxygen balanceassumes that all oxygen consuming reactions involve a soluble substrateunder biological growth reactions.

In a dispersed growth culture, new micro-organisms are formed as otherviable cells are lost through endogenous metabolism, lysis andpredation. The net active fraction of a bioculture is related to thelimiting fraction of non-degradables, sludge age (MCRT) and the loss ofcell viability. Reduction in the availability of food (the initialloading condition) or the over (extended) aeration of a culture havinglimited food availability will effectively cause a loss of microbialviability.

The transfer of dissolved oxygen to the liquid phase for use in meetingthe oxygen demand of the combined wastewater and bioculture is verycomplex. The most important factors that need to be considered include,the water chemistry, the specific geometry and mechanism of the transferdevice, basin geometry (width, length, side water depth), power inputper unit volume of wetted basin, wetted depth to wetted area of basin,total dissolved solids, residual dissolved oxygen concentration,temperature, surface tension, mean diameter of air bubbles, retentiontime of air bubbles in liquid medium, oxygen demand of basin contents,air flow rate per oxygen transfer device, ratio of areas of air flowrate device to total basin floor area, area distribution of oxygentransfer devices, altitude, concentration of the bioculture, systemsludge age, active fraction of the bioculture, mean particle size of thebioculture, bulk removal rate of dissolved oxygen by the biomass(hereinafter referred to as BIORATE).

Oxygen and its rate of utilisation, for all of the reactions takingplace involving the adsorption, absorption of nutrients, theirmetabolism into biosolids and the subsequent degradation of biomass, isof prime importance. The provision of oxygen at an adequate rate istherefore the key element to the use of cyclic aerobic, facultative andanaerobic micro-biological treatment methodology for the net rate ofremoval of nutrients by oxidative and reductive means, for the net rateof accumulation of biosolids and for the net removal of phosphates bybiological means. The rate of supply of oxygen, its net residualconcentration and the BIORATE, relative to the So/Xo distributiongenerally determines net growth factors for different groups ofmicro-organisms generally described as predominantly floc-forming or asfilamentous forms. An overgrowth of filamentous forms iscounterproductive to the treatment goals as this condition causes adisruption of the processing time scale for solids--liquid separation.it is therefore mandatory that biological growth associated withpredominantly floc-forming micro-organisms. The cognation of thepreferred process embodiment and the means for biomass process controlbased on oxygen utilisation rate set-points are directed to thisobjective.

The removal of nutrients by each of the mechanisms of adsorption,biosorption, oxidation and assimilation with ultimate aerobicdestruction of biological solids requires different oxygen fractions.The net use of oxygen is directly related to the proportion of nutrientremoval by each mechanism.

BIGRATE is a function of the condition of the biomass and the nature ofthe soluble substrate in contact with the biomass. A single sludgesystem can be made to exhibit a maximum biorate and a minimum bioratedepending on aeration time and the initial ratio of So/Xo. The activefraction of the biomass influences the range of biorate that its biomasswill exhibit.

Data taken from a 5 series complete-mix, constant volume reactor systemis presented to demonstrate typical magnitudes and changes that takeplace.

                  TABLE 3                                                         ______________________________________                                        Biorate and Associated Parameters                                                                      Biorate I                                            So/Xo            MCRT    mg                                                   mg      mg.sup.-1    d       O.sub.2 gvsshr.sup.-1                            ______________________________________                                        4.0              1       147                                                  1.0              2       90                                                   0.5              3       66                                                   0.25             8       56                                                   0.21             15      43                                                   0.21             40      35                                                   ______________________________________                                    

For these rates the initial reactor operated with a 70 minute residencetime and the total reactor 420 minutes residence time.

                  TABLE 4                                                         ______________________________________                                        So/Xo vs Biorate (mg O.sub.2 g.sup.-1 VSS h.sup.-1)                           ______________________________________                                        So/Xo 0.056  0.062  0.113                                                                              0.182                                                                              0.197                                                                              0.388                                                                              0.437                                                                              1.00 4.0                         Biorate                                                                             35.2   33.1   43.1 57.9 56.3 74.4 70.4 90.0 147                         ______________________________________                                    

Instantaneous oxygen utilisation rate can be typically measured by abench scale method in which the time concentration measurement ofdissolved oxygen depletion of an oxygenated sample of activated sludgeremoved from the process reactor is measured. This is a simple batchtest which requires a sample to be taken from the activated sludgereaction basin, aerated, placed in a mixed reactor into which is placeda dissolved oxygen measuring sensor; ingress of air is prevented. Whenthe dissolved oxygen meter senses oxygen depletion is taking place,measurements of dissolved oxygen versus tine are taken.

Respirometry control as it is currently practised in activated Sludgeprocessing is complex and indirect. Respiration rates are measured witha meter which typically consists of a closed completely mixedrespiration chamber through which activated sludge from the reactingaeration tank is continuously pumped. Dissolved oxygen concentration isperiodically measured with an oxygen sensor at the inlet as well as atthe outlet of the respiration chabber which can be achieved by alteringthe flow direction using a system of valves (as one method).

The problem with measuring the oxygen content at the inlet and outlet ofthe respiration chamber is that the oxygen content within therespiration chamber varies significantly from the oxygen content at theinlet and outlet of the chamber thus giving erroneous measurement.

The aim of this present invention is to provide a wastewater planttreatment and a method of treating wastewater wherein the metabolicactivity of the biomass is maintained at a level to ensure a maximumrate of biological removal of nutrients by oxidative and reductive meansthrough the measurement of BIORATE as previously specified within theprincipal reaction basin through measurements that occur by the sensingof oxygen concentration response changes at the end of an aerationsequence.

The wastewater treatment plant of the present invention comprises aprincipal reactor means capable of maintaining wastewater in contactwith biologically active degrading micro-organisms, a receiving means toreceive wastewater into the reactor means, an oxygen transfer meanswhereby air is introduced into the principal reactor, a control meansfor operating the said sequences and necessary equipment, an oxygendetection means to detect the relative changes in dissolved oxygenpresent in the principal reactor means and a control means to controlthe amount of oxygen introduced into the principal reactor means so thatthe activity of the micro-organisms is not limited by the amount ofoxygen present in the principal reactor wherein the oxygen detection iswithin the principal reactor means.

Measured in the biomass according to the present invention there isprovided an apparatus or a process using dispersed growth biologicalcultures for the treatment of wastewater which comprises the followingin combination with each other:

A means for maintaining a maximum potential BIORATE in an initialdesignated unaerated reaction zone for the culture through the definedadmixture of influent wastewater and biomass from the principal andfinal designated reaction zone, a means for introducing dissolved oxygeninto the specified principal reaction zone(s) for operation underpreselected area and pre-programmed aeration sequences, a means forinterrupting the influent wastewater to the initial designated reactionzone, a means for removing a fraction of the supernatant clear treatedwastewater after a set sequence of non-aeration, a means for detectingand measuring the position of the biosludge interface layer, a means forinteracting the biosolids interface with the biosludge wasting programwith the detection of the biosludge interface position, a means forsetting automatic time sequences for automatic operations, a means foroperation of the principal final designated reaction volume as avariable volume complete-mix unit, a means for measuring the biorate inthe principal final designated reaction volume using a dissolved oxygensensor properly placed in that basin volume, a means for measuring therate of change of dissolved oxygen concentration and making comparisonwith the actual respiration rate to control the rate of introduction ofdissolved oxygen into the treatment system, a means of operation formaximising the ratio of potential oxygen utilization rate (determinedthrough defined ad mixture of influent and biomass from the principalreactor) to oxygen utilisation rate in the principal reactor, a meansfor automatically setting the duration of the aeration sequence asmeasured and calculated by the actual respiration rate, a means foroptimising the use of aeration power to effect nitrification anddenitrification, a means of operating the system through BIORATE controlto effect maximum biological phosphorus removal, a means for operatingthe process so that the principal final designated aeration volumeoperates at an approximate biological steady state actual respirationrate (corrected for active fraction of biomass), a means for using thedissolved oxygen depletion rate that results from interrupting the airflow to the basin and a biomass concentration settling algorithm toprovide the BIORATE parameter, a means for removing near surfacesupernatant liquor at from about 20 cm below the liquid surface at aconstant rate to equivalent liquid depths up to two metres in apreferred 5-6 metre basin depth wherein the reactor configurationspermit end basin or across basin centre feed location, and the reactorconfigurations permit transverse or longitudinal location of effluentdecanting devices, whereby the apparatus and process is used to treatwastewater.

The wastewater treatment plant may consist of one or more reactors and aminimum of one principal reactor. In a preferred embodiment, thewastewater treatment plant consists of at least two reactors in fluidcommunication means. In one embodiment the plant consists of severalreactors in fluid communication wherein different components such asnitrogen, phosphorus, carbon and the like are together accumulativelyremoved in different reactors. In a further embodiment the oxygencontent in each reactor is significantly different.

In a particularly preferred embodiment the wastewater treatment plantcomprises at least two reactors, a first reactor with multiple zones,typically unaerated whereby absorption and biological phosphorus releasemechanisms take place, a second reactor which operates through cyclicaloxic--anoxic--anaerobic conditions for the microbial degradation ofcarbon compounds and TKN compounds in a wastewater and for the microbialremoval of NO₃ --N, No₂ --N and the microbial removal of P in thewastewaters; both reactors are in fluid communication.

In a further embodiment the waste treatment plant comprises oneprincipal reactor and the conditions within the reactor are adjustedcyclically so that the conditions vary from aerobic to anoxic toanaerobic and are repeated using definitions described previously.

The oxygen detection means may be any suitable means for detectingdissolved oxygen. Preferably the oxygen detection means detectsdissolved oxygen. More preferably, the oxygen detection means is anelectronic oxygen sensor able to measure the rate of change of dissolvedoxygen concentration as a 4-20 milliamp primary control signal throughthe use of a computer and other programmable logic controller throughwhich output signals are generated which allow interactive control ofthe rate of introduction of air into the reactor according to a setconcentration profile. More preferably the oxygen concentration issensed as a result of aeration of the wastewater/microbial mix in theprincipal reactor.

The oxygen concentration is typically adjusted during water treatment.Preferably the concentration of oxygen in the wastewater/microbial mixis adjusted during an aeration sequence. In particular, theconcentration of oxygen present is controlled by adjusting the durationof the aeration sequence and/or adjusting the flow of air in theaeration sequence. The flow of air may be controlled by a speed controlmechanism on the generator of the air supply flow or in the flow of airthrough a position control mechanism of a suitable control valve orother means that are specific to the oxygen input device. Control of theair flow by either means results in the control of the mass rate oftransfer of dissolved oxygen to the principal reactor.

The oxygen sensor is preferably located within the principal reactoritself. The oxygen sensor is located within the wastewater/microbialmix. More preferably the oxygen sensor is located around 30 cm away fromany surface of the principal reactor floor. Alternatively, the sensorcan be located in a pipe through which biomass from the principalreactor is pumped.

In one embodiment of the present invention the oxygen sensor calculatesthe actual in basin oxygen uptake based on the sum of the endogenous orbasic oxygen uptake and the oxygen uptake rate for oxidation of readilybiodegradable substrates, such as substrates in the carbon and nitrogenform, depending upon the micro-organisms that are present and theoperating sludge age of the system taking into account altitude andtemperature.

Experimental work has shown a relationship to exist between the ratio ofpotential oxygen utilisation rate and sludge settleability, provideddissolved oxygen concentration is not limiting. A further relationshipexists relative to the value of actual oxygen utilisation rate and therate of depletion of oxidation reduction potential. The value of actualoxygen utilization rate, over and above the endogenous oxygenutilisation rate also relates to a quantification of the mass of storedreadily degradable soluble substrate remaining in the biomass and thecapability of that biomass to participate in quantitative enhancedbiological phosphorus removal mechanisms. An embodiment of the inventionis to provide means of maintaining a mass transfer of oxygen (throughaeration) which approximately equates to the biomass oxygen demand andby such means cause the aerobic degradation mechanisms to take place atan optimal use of oxygen transfer energy. Automatic means are providedfor setting the length of the aeration sequence, the mass ofmicro-organisms to be carried in the principal reactor, setting thedesirable dissolved oxygen concentration profile in accordance with theresultant set-point oxygen utilisation rate measured at the end of theaeration sequence and the magnitude of the POUR/OUR ratio.

That the embodiment of the invention is such as to cause co-currentnitrification-denitrification to essentially practical completion and toprovide for biologically enhanced phosphorus removal mechanisms that arewell known to those experienced in the art.

In one embodiment there is one or more reactors the first receives an influid communication one of which is admixture of wastewater andmicro-organisms contained in the mixed liquor from the last reactor.

In a preferred embodiment the invention relates to the use of afed-batch reactor volume which is essentially operated as a completelymixed reactor during an aerated sequence, albeit of variable volume,during which time a combined flow of influent domestic wastewater and aflow of mixed liquor solids from the fed-batch reactor volume isintroduced.

Even more preferably, a wastewater/microbial mix goes through a completeaeration cycle. The same mix then undergoes a non-aeration cycle, duringwhich time a solids layer and an upper supernatant layer segregate. Thesequence of events are completed through the removal of a fraction ofthe upper supernatant layer from the principal reactor using decantingmeans. The whole cycle is then repeated.

Control and measurement of the respirometric capacity of the biomassdirectly in the principal reactor is made possible through thecomplete-mix air-on and air-off operation that takes place in thepreferred variable volume activated sludge treatment methodology. It isalso possible to check the progress of treatment in an aerated reactionsequence through interruption of the air flow and subsequent measurementof the dissolved oxygen depletion rate.

Measurement of the end of sequence oxygen utilisation rate, combinedwith the comparison of received process volume (versus minimum set-pointvolumes) provides the basis for automatic in sequence aeration cycleadjustment which effectively increases the organic loading and henceoxygen utilisation rate as an assurance for biological phosphorusuptake, following its release during otherwise unfavourable uptakereaction conditions.

state-of-the-art on line respirometry as typically applied measuresdissolved oxygen concentration in the outlet of a respirometer chamberseparate from the principal activated sludge reactor, which is equal tothe dissolved oxygen concentration in the respiration chamber and shouldnot be rate limiting. If necessary the activated sludge should beaerated before it enters the respiration chamber. The respiration rateis typically measured every minute from the mass balance of dissolvedoxygen over the separate respiration chamber. The actual respirationrate is defined as the oxygen uptake rate in the principal aerationtank. To measure this rate, activated sludge from the principal aeratedreactor is continuously pumped into the on line separate respirationchamber which is equal to the mean actual respiration rate in theprincipal activated sludge reactor basin provided that the sludgeloading in the respiration chamber equals the loading in the aerationtank. To maintain loading equivalence influent is continuously added tothe sludge flowing into the respiration chainber in the proportion.

Qsam=Qin Vres/Vat

Qsam=influent sample flow to respiration chamber.

Qin=influent flow

Vres=volume respiration chamber

Vat=volume aeration tank

In all cases on-line respirometry is measured in a scaled down versionof the organic loading conditions that exist in the main aerated reactorof an activated sludge plant. A number of simple respiration rates areso identified; the endogenous respiration rate which is typicallydefined as the oxygen uptake rate of activate sludge that has beenaerated for 1.5 hours without feeding. The maximum respiration rate isdefined as the oxygen uptake rate of activated sludge with an excess ofsoluble substrate (readily biodegradable matter). This rate is measuredwhen an excess of influent is continuously introduced to the sludgeflowing into the respiration chamber. The instantaneous respiration rateis defined as the oxygen uptake rate of activated sludge flowingdirectly from the completely mixed aeration tank through the respirationchamber. The rate is typically lower than the oxygen uptake rate in theaeration tank the actual respiration rate. The absolute value of theinstantaneous respiration rate depends on the detention time in therespiration chamber. The maximum respiration rate of a biomass is alsoequivalent to its potential oxygen utilisation rate.

The embodiment of the present invention uses actual respirometric ratecontrol from measurements taken within the aeration reactor (theprincipal reactor), not from an inline separate detection unit as is thecurrent general practice.

The actual respiration rate in the preferred embodiment of the inventionis the sum of the endogenous or basic respiration and the uptake ratefor oxidation of readily biodegradable substrate, both carbon andnitrogen forms, the latter only occurring if a nitrifying biomass isselectively grown. At maximum respiration rate the activated sludge willbe in an overloaded condition and will result in incomplete removal ofreadily biodegradable substrate. This means there is a criticalrespiration rate in between maximum and basic respiration rate and atthis rate the effluent quality meets the requirements and the removal ofreadily biodegradable substrate, among other parameters is satisfactory.At no time should the oxygenation capacity be rate limiting. It isnecessary that the kinetic processes that utilise dissolved oxygen becomplete to within the reaction time that is provided for the completionof those reactions. In the case of nitrifying mechanisms, thetransferred oxygen required by the oxygen demand must be satisfied bythe oxygen supply--time relationship indicated and provided by therespiratory measurement. It is necessary to initially determine bymanual means, loading rates, actual respiration rates and dissolvedoxygen concentration. There is an advantage when the actual respirationrate is always equal to or near to the critical actual respiration rate.In this case the activated sludge is never overloaded and works at amaximum acceptable rate. Therefore the total amount of activated sludgemaintained in the system is optimal and the metabolic activity of thebiomass can be maintained at acceptable values to assist with othernutrient removal reactions. An ideal constant actual respiration ratecan always be met through manipulation of biomass concentration,aeration time, and rate of supply of oxygen demand.

To those exprienced in the art, there are a number of ways of operatingdispersed growth wastewater treatment systems. These generally includethe operation of one or more connected reactors, at constant volume atleast one of which is aerated continuously, through which the admixtureof wastewater and micro-organisms flow. The final basin in these systemsis a "quiescent" non-aerated vessel in which solids liquid separationtakes place, the clear overflow supernatant being the treated effluentand the underflow solids which are directed to waste and to the reactantvessels. Various internal recycle flows also occur. While the inventioncan be embodied in this configuration, it is not so limited in itsapplication. In its preferred embodiment the invention relates to theuse of a fed-batch reactor volume which is essentially operated as acompletely mixed reactor during an aerated sequence, albeit of variablevolume, during which time a combined flow of influent wastewater and aflow of mixed liquor solids from that reactor is introduced.

This invention, in its preferred embodiment is specific to reactionconditions that are generated and not necessarily to numbers and zonesof the reactor volumes through which the said reactants pass. This isnot a limitation on the embodiment. Principally the volume fraction asdescribed as the fed-batch reactor undergoes complete mix aeration,during a specific aeration cycle, for which variable volume complete mixkinetics can be ascribed to that specific volume. Following the specificnon-aeration sequence, during which time a solids layer and an uppersupernatant layer segregate, the relative depths being dependent uponthe contact flow history of influent wastewater and the mixed liquorsolids concentration of a stream of solids, which is directed from theprincipal variable--volume completely mixed volume to the influentstream of wastewater for admixture. This embodiment of operationrequires a means of removing a specified fraction of the supernatantupper layer during the continued non-aeration sequence. When this eventis completed, the aerated sequence is continued with further admixtureof reactants as prescribed previously.

While not limiting the embodiment of the invention, t he mode ofoperation of fed-batch reactor treatment methodology is most easilyconducted in more than one basin module. Cycles of aeration operationcan be easily set for 2 hours and other two basin multiples. Othercycles of operation can be set for 3 basins, and other additions, foreither even or odd basin operation. The embodiment of the inventionwhile not limited to the basin modules, is easily explained as a twobasin operation. Those experienced in the art will be able toextrapolate from the two basin operation used in this discussion.

Whilst upstream reaction volumes have an important bearing on theefficiency of the treatment methodology, the principle requirement isthat there is a major volume fraction of the fed-batch reactor volume,in excess of 50%, that undergoes variable volume complete mix reactionconditions, using a specific device for combined aeration and mixing.

While it is preferable that a system of diffused aeration is used, thisdoes not necessarily limit the application of the invention. Two set-upsfor the invention will be described. Both configurations require the useof a dissolved oxygen sensor having an acceptable response time formeasuring a rate of change of dissolved oxygen concentration (do₂ /dt).

Previous discussion has explained the importance of load demand and loadsupply of dissolved oxygen, relative to substrate load, load applicationtime and viable fraction of biomass.

The first configuration requires the use of a suitable dissolved oxygensensor, complete with the electronics that are necessary to enable themeasurement of the rate of change of dissolved oxygen concentration as acontrol signal, through the use of a specific computer or otherprogrammable logic controller, through which output signals aregenerated, which allow interactive control of the rate of introductionof air into the complete-mix reactor (and/or other fluidly connectedreactor volumes), during the aeration sequence. Interactive control isthrough the duration of the aeration sequence combined with the flow ofair through a speed control mechanism on the generation of the airsupply or in the flow of air through a position control mechanism of asuitable control valve, as a means of restricting the flow of air.Control of the airflow by either means results in the control of themass rate of transfer of dissolved oxygen to the complete-mix fed batchreactor.

In the first preferred embodiment the invention requires a minimum ofone reactor vessel, preferably operated as a fed-batch reactor, whichoperates as a variable volume activated sludge reactor basin. During theprocess of filling and aeration where more than one vessel compartmentis used, these will be in fluid communication.

An important feature of the invention is the manner and means by whichthe wastewater to be treated is introduced into the means for reaction.Also important is the initial mass ratio of activated sludge solids thatis caused to come into contact with the influent waste flow. Of furtherimportance is the time of interaction of these component flows and themeans by which intermixing and intermeshing of the two flow streams ismaintained. One method employed in the industry utilises either fixedsub surface or floating surface electrically operated propellers whichcause a directional flow to take place and an intermixing of solids andliquid phases through the expenditure of energy. The invention can beused with this means of operation. The preferred embodiment of theinvention contains no specially installed equipment of the type referredto. Mixing in this invention is variously caused through the operationof the means for aeration, which is essential to the aerobic degradationand anomic degradation processes that are maintained and or the designof combined flow conditions using conduits, channels and flowdirectional baffles.

It has been found that there are benefits in process that derive fromthe means of introducing the relative proportions of activated sludgesolids and wastewater, the time of flow-mixed contact of these twostreams and the manner in which kinetic natural mixing is used duringthe initial contacting reaction period. While not omitting theapplication of the invention, the combined initial reaction time isdesigned to ensure a minimum of 65% removal of the readily degradablesoluble substrate fraction contained in a wastewater. This fraction canbe variable in wastewaters. By way of example, for a BOD of around 300mg/l, Aand an associated COD of around 600 mg/l, in a domesticwastewater, and for a typical reticulation design, a 25% readilydegradable soluble substrate fraction assumption base will giveacceptably good process results. A process reaction time of twenty toaround sixty minutes hydraulic retention tiae within the biologicalselector will normally generate the desired result, provided thecopartmentalisation required of the inlet configuration design performswith the correct degree of dispersion together with an appropriatemixing energy that enhances biological floc nucleation and aggregation.The relative placement of overflow and underflow baffles relative todesignated bottom water level and the reactor basin floor is a featureof the invention. The open area of the underflow baffle is restricted togenerate a high underflow energy which is more than three times greaterthan the mean flow energy across the overflow weir. The underflow freearea uses a fraction of the available length of the underflow baffle.Thus high mixing energy regimes are generated near to the reactor basinfloor sections which are followed by reduced energyfluctuation-aggregation zones at the upper zone, formed by the overflowbaffles. The inlet configuration geometry in designed to promote pulsedenergy zones which ensure floc transport and floc growth, together withthe biological reactions of soluble BOD removal and conversion tointracellular storage products, partial denitrification and phosphorusrelease by the biological phosphorus sequestering micro-organisms thatare caused to grow in the biomass.

While all of the processes referred to above take place in a singlevessel embodiment, a preferred embodiment utilises a four (4) basinfacility or a four (4) module facility. each module can comprise one (1)to N (where N≧1) basin combinations. The factoring on 4 modules isdependent on the set (design) four (4) hour cycle upon which the basingeometry is designed. To those experienced in the art it is obvious thatother factoring numbers such as 3 and 5 can equally be used. Such designsatisfies specific requirements for load hydraulic) division, organicload manipulation, biological treatment (including concurrentnitrification-denitrification and biological phosphorus removal)provision of oxygen demand by automatic biorate control, maximisation ofoxygen transfer efficiency, optimisation of solids-liquid separationrelative to the decant depth and decant removal rate of treatedeffluent. The four module preferred embodiment operates in every way asa net continuous process, with acceptance of influent on an as receivedbasis with a continuous discharge of effluent from the plant, the flowrate being an hourly constant rate relative to the actual decant volumethat is removed from each module. A different protocol can be runwhereby the discharge rate is constant at each decant sequence. Thepreferred embodiment is configured for a flow split operation followedby the four module (basin) processing. A module can be configured withinfluent at one end of the module (basin) and effluent decanting at theopposite end or with effluent decanting at the remote end of the module(basin) but located on the long basin walls (see FIGS. 6(a) to 6(g)).Typically a domestic wastewater containing 300 mg/l TSS, 55 mg/l TKNwhich is to be treated to a flow range of 6×ADWF will require an inletconfiguration zone of up to 8% of the total vessel area. This zone isdivided into a minimum of 5 and typically between 8 and 14 sub zones foreach principal reactor each having a volume fraction that initiallygenerates an oxygen uptake rate in the first mixed zone of in excess of20 mg O_(2/) gVSS/hr. The volume fraction of mixed liquor suspendedsolids from the main reactor volume will typically be in excess of 20%and less than 33% of average influent flow. Under overflow bafflearrangement terminates on either side of the reactor basin such thathalf of the combined flow discharges to a position on either side of theprincipal reactor basin.

Pumped mixed liquor suspended solids continues throughout the durationof the complete cycle. Influent wastewater is interrupted during thesettle sequence. Waste sludge is collected from zone subsequent to theinlet configured biological selector, and removed during an aerationsequence or during the non-aeration settle sequence. Reactor basindimensioning is typically based on up to 15 kg MLSS/m² of reactor area;and for efficient nutrient removal in domestic wastewater, a BOD load of0.33-0.40 kg BOD/m³ at a fractional decant volume of 0.46. Decant liquiddepth removal is up to 38 mm/min. without the addition of phosphorusprecipitant. With the addition of phosphorus precipitant, for normal dryweather treatment operation this depth rate can be increased to 44mm/min. Basin solids flux load is up to 15 kg MLSS/m² and up to 10 kgTKN/kg MLSS/m² /d, within 20% for the former and within 30% for thelatter.

A further development of the system incorporates attached growth mediato enhance the volumetric biomass load that can be accommodated in thesystem. For this embodiment the variable volume reactor basin is dividedinto three zones.

The first is the biological selector zone which is sized for domesticwastewater generally as per the above description. For organicindustrial wastewaters this fraction is increased to occupyapproximately 12% of the basin surface area. The zone iscompartmentalised as described to effect successive removal of solublesubstrate. The first zone is followed by a second zone in fluidcommunication. The return flow of mixed liquor solids from zone 3 tozone 1 for applications where the influent BOD is up to 2000 mg/l orzone 2 to zone 1 increases to two to three times the average influentflow. the caged random pack media is contained in a flow through cage.Zones 1 through 3 are in continuous fluid communication. Random packingin zone 2 is approximately 0.4 metres from the reactor basin floor andto within 0.15 metres below designated bottom water level. Zone 2 isfitted with means for varying aeration intensity, zone 1 has aerationdiffusers connected to valves which allow coarse aeration/mixingcontrol.

It will be obvious to those experienced in the art that the same mode ofoperation and control applies to the treatment of wastewaters for carbonremoval only, for carbon and nitrogen removal, for carbon and phosphorusremoval and for carbon and nitrogen and phosphorus removal.

The described arrangement has been advanced by explanation and manymodifications may be made without departing from the spirit and scope ofthe invention which includes every novel feature and novel combinationof features hereindisclosed.

Those skilled in the art will appreciate that the invention describedherein is susceptible to variations and modifications other than thosespecifically described. It is understood that the invention includes allsuch variations and modifications which fall within the spirit andscope.

I claim:
 1. A method of treating waste material forming at least a partof a biomass comprising a single activated sludge in a variable depthbioreactor using controlled intermittent and successive aerationsequencing and liquid decantation to concurrently grow and maintain aculture of autotrophic, heterotrophic and facultative micro-organisms inthe sequentially aerated single activated sludge for the biologicalremoval of the organic carbon, nitrogen and phosphorus components fromwastewater admitted to the bioreactor, said biomass being located in avariable depth operated reactor having at least two interconnected zonesin series connection in which one of the zones is a first reaction zoneand the other zone in a second or last zone, wherein at least a part ofthe treated contents of the second zone of the reactor is recycled to apartially segregated non-aerated volume of the first reactor zone foradmixture with incoming influent waste, at least during an aerationsequence of operation of the second or last of the variable depthoperated reactor, wherein the method comprises using one dissolvedoxygen concentration sensor or probe means for automatically andcontinuously monitoring dissolved oxygen concentration in the biomass inthe second or last zone of the variable depth reactor, said sensor orprobe means being located in the biomass at a location such that atleast that part of the biomass in that location is in motion during thetime of automatically and continuously measuring the dissolved oxygenconcentration, whereby the single sensor or probe means is used to causeoperation of an oxygen input means during input into and aeration of thewastewater in the second or last zone, in combination with computermeans to operate algorithms in order to operate to a set protocol ofsuccessively increasing dissolved oxygen concentration from zero toabout 2.5 Mg/L in discrete predetermined adjustable time increments tooptimize the retention of adsorbed organic substance within the biomasswhile maintaining co-current and optimal nitrification anddenitrification during aerated operation, with phosphorus release duringnon-aeration and phosphorus uptake during adjacent and reactive influentaeration sequences, with the detection and automatic calculation of theoxygen utilization rate of that biomass in the second or last variablevolume zone which adjusts the length of each aeration sequence exposureof the biomass, said determination and adjustments being characterizedby the biomass in the second or last zone of the reactor having apotential oxygen uptake rate, measured using an aerated admixture of80%/20% single sludge biosolids/influent mixture, being in excess ofabout three times the measured uptake rate of the single sludgebiosolids as measured by the single dissolved oxygen sensor, such thatcombined with the preset oxygen transfer rate and the potential oxygenuptake causes a limitation to the nitrogen oxidation product toessentially nitrite nitrogen form, and to cause by aerated mixing in thesecond or last variable volume zone a concurrent reduction reaction ofthe nitrite nitrogen to essentially nitrogen gas, in such a way that atthe end of the aeration sequence, the biomass oxygen utilization rate isautomatically controlled to an operating set point, adjunctively withthe introduction of air into one or more partially segregated volumeswithin the first zone of the reactor to partially limit the release ofphosphate in the biological phosphorus removal mechanism, such that thefirst zone of the biological reactor can be continuously andautomatically controlled to limit oxic, anoxic and anaerobic successivereaction environments in the first zone of the variable depth biologicalreactor.
 2. The method of claim 1, wherein the waste is domestic,industrial or commercial wastewater, including human wastes, bodywashing wastewater, clothes washing wastewater, food preparationwastewater, and combinations thereof.
 3. The method of claim 2, whereinthe last reaction zone is more than 50 percent of the total reactionvolume, and the first zone receives mixed or unmixed contents, recycledfrom the second or last reactor zone for admixture with incoming waste.4. The method of claim 1, wherein up to 40 percent of the design depthof the variable depth reactor is removed during the decantation step ata rate that does not cause removal of settled solids from within asettled sludge layer in the reactor.
 5. The method of claim 1, whereinthe second or last reactor zone is provided with oxygen transferdiffusion grids located at or towards the floor or base of the principalreactor.
 6. The method of claim 5, wherein the bioreactor is providedwith at least one air supply line provided with at least one motoroperated control valve, so that the motor/operated control valve(s) canbe alternately opened for a set program of air-on operation in a cycleand then closed.
 7. The method of claim 6, wherein all of the motoroperated control valves are operated in unison during the aerationsequence, or some of the valves are closed, or all of the valves areopened and closed according to a preset sequence of operation.
 8. Themethod of claim 1, wherein the net fluid oxidation reduction potentialof the combined liquid stream passing through the initial reaction zoneobtains a value of less than about -150 mV as compared to a hydrogenreference electrode.
 9. The method of claim 1, wherein up to 40 percentof the total bioreactor volume is introduced into the first zone duringa time which is equivalent to the cycle time less the liquid removalair-on/off time sequence.
 10. The method of claim 1, wherein the-cyclicair-on time exposure of the biomass and the amount of recycled treatedwaste admixed with the influent wastewater is sufficient to yield a lessthan -150 mV oxidation reduction potential in a time of less than 80minutes.
 11. The method of claim 1, wherein the oxidation reductionpotential of segregated sludge in the second or last reaction zone fallssubstantially to less than -150 mV within 90 minutes into the air-offsequence.
 12. The method of claim 1, wherein the solids concentration ofthe biologically activated sludge of the second or last mixed reactionzone is up to about 5000 mg/L.
 13. The method of claim 1, wherein thebioreactor is formed with vertical walls of reinforced concrete orstructural steel or formed as a slope walled lagoon structure havingearthen, concrete stabilized, membrane lined or concrete retainingwalls.
 14. The method of claim 1, wherein the biomass remains in motionfor up to 10 minutes after interruption of-the supply of air or oxygen.15. The method of claim 1, wherein the values of dissolved oxygenconcentration are automatically sensed and monitored in situsubstantially continuously but not less than at intervals of 10 to 20seconds during the total air-on and air-off sequences of each cycle. 16.The method of claim 1, wherein the use of the cycles of operation aremanaged by the measurement of the oxygen utilization rate in order toadjust it to appropriate values to provide for the satisfaction ofreactor stoichiometiric oxygen demand which permits a single air supplyto service one or more two zones of the bioreactor.
 17. The method ofclaim 1, wherein the dissolved oxygen concentration sensor or probe isan electronic oxygen sensor able to measure the rate of change ofdissolved oxygen concentration as a 4-20 milliamp primary controlsignal.
 18. The method of claim 17, wherein the oxygen sensor is locatedwithin the second reactor about 30 cm from the surface of the secondreactor floor, or in a full-flow conduit or pipe through which part ofthe liquid/solid material from the second reactor flows to the influentadmission reactor.
 19. The method of claim 1, wherein the TKN loading onthe activated sludge is up to about 0.01 kg TKN/kgMLSS/M² /d for typicaldomestic sewage applications.
 20. The method of claim 1, wherein thetotal phosphorus loading of activated sludge solids is up to about 0.002kg Phosphorus/kgMLSS/M² /d for typical domestic sewage applications. 21.The method of claim 1, wherein the dissolved oxygen concentration in theprincipal reactor is controlled to less than 0.7 mg/L (average) for 75percent of the air-on time and to between 2 and 3 mg/L for the remainingair-on time period.
 22. The method of claim 1, furthercomprising:microbially treating the wastewater in the presence of amicro-organism population acclimated to the wastewater contaminants andtheir concentrations in the wastewater, said micro-organism including,nitrifying micro-organisms capable of converting nitrogen to at leastnitrite nitrogen, facultative micro-organisms capable of denitrifyingnitrite and optionally nitrifying organisms capable of convertingnitrite to nitrate nitrogen, and facultative micro-organisms capable ofreducing nitrate to nitrite nitrogen to nitrogen gas and phosphorusremoval micro-organisms capable of biologically removing availablesoluble phosphorus.
 23. The method according to claim 1, wherein themixed liquor solids concentration in the second or last reactor issensed and recorded at the moment that the air supply to that reactor interminated and the oxygen uptake rate is sensed, recovered and analyzedfollowing termination of the process oxygen supply and the liquid levelat the time of closure of the influent valve to the reactor plus twominutes.
 24. The method of claim 23, wherein the sensed process valuesare processed and used to determine:the waste sludge pumping time, theduration of the air-on sequence for the next cycle, the mass flow rateof air for the next cycle, adjustment of the dissolved oxygenconcentration set-points, such that the process conditions aresufficient to maintain the set-point oxygen uptake rate in the principalreactor determined at the end of the previous aeration sequence.
 25. Themethod of claim 1, wherein a pH correction is made to the influentwastewaters.
 26. The method of claim 1, further including a flow path ofadmixed components from the first zone of the bioreactor wherein theflow path has successive passes from adjacent the reactor floor to theliquid surface of the reactor in transit to an adjacent zone of thebioreactor, in which the mixing energy associated with the flow pathnear the reactor floor of the first reactor compartment is a minimum of3 times the mixing energy associated with the flow path near to theliquid surface in succession thereby causing localized energy pulsation,nucleation and flocculation of the admixture.
 27. The method of claim 1,wherein the set-point oxygen uptake rate is experimentally determinedand is up to 20±4 mgO₂ /gVSS/hour (referenced to 20° C.).
 28. The methodof claim 1, wherein in which there are four bioreactors, or four modulesforming the bioreactor and a flow splitter arrangement for distributinginfluent waste to each bioreactor or each module wherein each bioreactoror each module functions an a single bioreactor.
 29. The method of claim28, wherein each bioreactor comprises an influent position, configuredinflow admixture compartments and an effluent decanting devicecomprising a moving liquid receiving channel designed to exclude surfacefloating material to effectively remove up to 40% of the bioreactordepth.
 30. The method of claim 28, wherein the oxygen uptake rate ormeasured potential oxygen uptake rate in the initial admixture reactoris at least 20 mgO₂ /gVSS/hr.
 31. An apparatus for biologically removingcarbon, nitrogen and phosphorus from wastewater, in the form of apartially enclosed water-retaining, multi-zone, variable-depth,cyclically-aerated reactor comprising at least a first hydraulic zoneand a last hydraulic zone separated by a partial wall structure allowingfluid communication and transfer between the zones at least during apart of an air-on sequence, an aerator for selectively exposing thecontents of the reactor to repeated air-on and air-off sequences, saidfirst hydraulic zone provided with an inlet for introducing influentwastewater to the first zone during at least the air-on sequence, saidlast hydraulic zone for allowing separation of the wastewater into atleast supernatant clear liquor, an aerator including a grid air bubblegeneration system for providing combined mixing and oxygen transfer inat least the last hydraulic zone mounted on the floor of the reactor anda means for directing a flow of process air to the reactor forin-reactor oxygen transfer at least two different mass flow rates duringthe air-on sequence, means for interrupting the flow of influentwastewater to the first hydraulic zone at least during a part of theair-off sequence, means for removing liquid contents from the lasthydraulic zone to a position remote from the reactor during at least theair-off sequence, means for transferring the contents from the lasthydraulic zone to the first inlet hydraulic zone at least during theair-on sequence, means for interrupting influent wastewater flow and theflow of process air to the reactor during at least a part of the air-offsequence, means for reducing the amount of supernatant clear liquidretained in the last hydraulic zone during the air-off sequence to apreselected lower level using a motor-driven decanter comprising ahorizontal weir box, fitted with a positive floating solids excludingscum guard, connected by at least one downcomer member to a rotatingdrum shaft provided with liquid retaining seals and airlock releasepipes, means for automatically maintaining an optimum mixture of processacclimated heterotrophic, autotrophic and facultative micro-organismsand wastewater through the continuous measurement of the rate of changeof dissolved oxygen concentration in the reactor together withmeasurement of the potential oxygen utilization rate of the biomass,said rate change of dissolved oxygen being measured by a singledissolved oxygen sensor located in the biomass, such that at least partof the biomass is in motion at the time the measurement is taken inorder to provide an indication of the utilization rate as a function oftime, a means for analyzing successive rates of change of oxygenconcentration taken at the end of each air-on sequence in the lasthydraulic zone, a means for continuously measuring the rate of change ofdissolved oxygen concentration at the beginning of each air-on sequence,means for adjusting set point operating positions of the rate of changeof dissolved oxygen in the last hydraulic zone of the reactor takinginto account process air flow rate, air-on time adjustment and mixedheterotrophic, autotrophic and facultative micro-organism culture, meansfor adjusting and operating with at least four set point positions forthe dissolved oxygen concentration as a function of time profile in eachair-on cycle in the last hydraulic zone in order to achieve anindication of the termination of the air-on sequence set point dissolvedoxygen concentration rate of use, means for automatically adjustingtheoperating time duration of each total cycle and successive cycle,means for operating with and determining the duration of air-offsequence time in successive cycle times in the last hydraulic zone,means for operating with and determining the time-based flow rate ofprocess air introduced to the reactor, and means for determining andexecuting the time of operation within each cycle for the removal of apredetermined volume of a mixture of biomass and wastewater insuccessive air-off sequences from the reactor.
 32. The apparatus ofclaim 31 further including computer means for carrying out the processof claim 1.